According to IUPAC nomenclature, what is the three-letter temporary...

The atomic number of the element is $120$.

According to IUPAC systematic nomenclature, the roots for the digits are:
$1 \rightarrow$ un
$2 \rightarrow$ bi
$0 \rightarrow$ nil

Combining these roots, the temporary IUPAC name for the element is unbinilium.

The three-letter temporary symbol is derived from the first letters of these roots, which gives Ubn.

Answer: Ubn